A manufacturing process for a semiconductor device or a liquid crystal display device typically includes a film-forming process or a photolithography process to form a circuit pattern on a substrate such as a wafer or a glass plate. A wet process which is performed during the above processes and uses mainly liquid employs a spin treatment apparatus to subject the substrate to treatments such as a chemical treatment, a washing treatment, and a drying treatment. A spin treatment apparatus is an apparatus configured to perform a wet process by clamping the outer peripheral surface of a substrate, rotating the substrate about an axis perpendicular to the substrate at the center thereof, and supplying a treatment liquid (e.g., a chemical liquid or pure water) to the rotating substrate. Such a spin treatment apparatus generally includes a chuck mechanism configured to chuck the substrate.
As this chuck mechanism, multiple clamp pins are provided along the periphery of a substrate to clamp an outer peripheral portion of the substrate. The clamp pins are each integral with a rotary plate and a rotary shaft body, and provided on the rotary plate at a position offset by a certain distance from the rotation axis of the rotary shaft body. Pinions are fixed to the lower ends of the respective rotary shaft bodies, and mesh with a master gear whose rotation axis is perpendicular to the substrate at the center thereof. Hence, when the master gear rotates, the individual rotation shaft bodies rotate, causing the clamp pins to eccentrically rotate. The substrate can be clamped by these clamp pins.
Besides such a gear-type chuck mechanism, a magnet-type chuck mechanism has also been developed. In this magnet-type chuck mechanism, a circular plate is provided at the lower end of each rotary shaft body in place of the pinion described above, and the circular plate has a magnet perpendicular to the rotary shaft body. Bringing another magnet close to this circular plate causes the magnet on the circular plate to rotate with the circular plate due to the attractive force of the other magnet. The rotation of the magnet rotates the rotary shaft body, rotating the clamp pin eccentrically. This magnet-type chuck mechanism is capable of clamping the substrate with the clamp pins like the gear-type chuck mechanism. To unclamp the substrate, a magnet different from the other magnet described above is brought close to an unclamp position which is opposite from a clamp position described above.
However, these chuck mechanisms have their drawbacks. Specifically, the gear-type chuck mechanism reliably performs chuck operation with the clamp pins, but might generate dust due to wear of the gears. The magnet-type chuck mechanism generates no dust, but has the following problem. Specifically, due to the configuration in which the attractive force of another magnet causes the magnet on the circular plate to rotate with the circular plate, the distance between the magnetic poles attracting each other varies during the rotation of the magnet. Since the attractive force between the magnetic poles is inversely proportional to the square of the distance between the magnetic poles, decrease in the distance between the magnetic poles drastically increases the attractive force therebetween. For this reason, when the distance between the magnetic poles fluctuates, the clamp pins tend not to rotate uniformly. The positioning of the substrate is performed based on a single one of the clamp pins which has the smallest distance between the magnetic poles, and such non-uniformity in the pin rotation might cause displacement of the substrate from a predetermined position when clamped. It is therefore demanded to suppress both the generation of dust and the displacement of a substrate when clamped.